1. Field of the Invention
This invention relates to a single-lens reflex type video still camera, and more particularly to improvement of its optical system.
2. Description of the Prior Art
A video still camera has an image sensing element and an optical low-pass filter arranged in front of said element. The optical low-pass filter is indispensable to prevent moire which will be produced inevitably without such optical low-pass filter due to the structure of the image sensing element. In projecting a black and white image onto the image sensing element, for example, if an image having a spatial frequency close to the picture element pitch of the CCD in the image sensing element is projected on the image sensing element, moire generated. When a color image is projected, moire is also generated when the image component having the spatial frequency close to the pitch of the stripe filter for color dissolution of the CCD in the image sensing element is projected.
The optical low-pass filter is to cut off the component of the spatial frequency which causes moire as described above, and is composed of double refraction material such as a crystal and fixed in front of the shutter mechanism arranged in front of the image sensing element.
Such a video still camera constructed in a single-lens reflex type has been introduced in ITE Technical Report Vol. 8 No. 35, TEBS100-4, December 1984. As shown herein by FIG. 14, the conventional single-lens reflex type video still camera includes a camera lens 2, an optical low-pass filter 8, a shutter mechanism 3 and an image sensing element 4 along the optical axis, wherein a finder mirror 5 and a half mirror 7 for direct light measurement are arranged between the camera lens 2 and the optical low-pass filter 8. A light bundle passing the lens 2 is reflected by the finder mirror 5 and focused into an image on a focusing plate 9. Thus, the image on the focusing plate 9 is observed visually through a pentaprism 10 and an eyepiece 12. When a shutter release button (not shown) of the camera is depressed, abovesaid finder mirror 5 is driven by a finder mirror driving mechanism (not shown) and goes up to a position shown at 5' outside of the photographing optical path. On the other hand, an aperture diaphragm 1 is stopped down to a predetermined aperture value by an aperture driving mechanism (not shown). The light bundle passing this aperture diaphragm 1 gets through the half mirror 7 and the optical low-pass filter 8 and is projected on an opening part of the shutter mechanism 3. A pair of shutter curtains (not shown) of the focal plane shutter mechanism 3 is operated after the operation of abovesaid finder mirror driving mechanism and the aperture driving mechanism is completed, and controls predetermined exposure time, and exposes the light bundle coming from aforesaid object to be photographed on a charge coupled device (CCD) 4a of the image sensing element 4. Besides, the half mirror 7 reflects a part of the light bundle which passed the aperture diaphragm 1 toward a light receiving element 11b.
However, in order to form such a construction as described above, it is required to provide a space to also locate the movable finder mirror 5 as well as the optical low-pass filter 8 between the rear end of the camera lens 2 and the image sensing element 4. This requires an increase in the back focal length of the camera lens system. Thus, the optical system becomes large in size and a compact single-lens reflex type video still camera is not obtainable.